There exists a need in industry for corrosion resistant-clad products, and, particularly, for clad pipes. These products are used in power plants, in processing plants, in downhole and line applications, in geothermal wells and high temperature, sour gas wells.
Clad pipes usually combine the required properties of a more expensive corrosion-resistant alloy (CRA) on the inside with the excellent strength, ductility, weldability and low costs of a carbon steel outer shell or host. The two materials are joined to form a metallic bond. A good metallic bond insures high strength of the clad product and offers optimum corrosion resistance.
Several manufacturing processes are known in the art for the manufacture of clad metal pipes. In one method, roll-clad metal sheets are bent to form a pipe, which is then longitudinally seam welded. However, this process must be carefully monitored and for many applications engineers prefer to limit the number of welds as much as possible. In a second method, known as centrifugal casting, a molten CRA metal melt is cast into a molten steel metal pipe which is mounted on bearings in such a way as to be rotatable about its axis. The molten metal is distributed evenly along the length and across the circumference of the metal pipe during its rotation. In the method of internal-pressure plating, an internal thin-walled pipe, which forms the internal cladding, is inserted accurately to fit into a metal pipe, and, with the application of hydraulic or pneumatic pressure, expanded onto the inner circumferential surface of the host pipe. (See, e.g., German Patent No. 4,406,188).
A more recent technique for manufacture to date has been to use an activator between the component layers. This initiates a liquid state diffusion between the corrosion resistant alloy and the host during extrusion, which is responsible for the metallurgical bond. Such a method is described in U.S. Pat. No. 4,620,660 (Turner). In this patented method, the internal surface of the host is plated with a low melting point bonding metal alloy. A cylindrical cladding member is placed inside the plated cylindrical host, and then circumferentially welded on one end to the host. The other end is then welded to a manifold with an evacuation tube and connected to a vacuum source. The annular space between the cladding member and the host is evacuated to remove water vapor and oxygen and sealed, which promotes diffusion across the interface during extrusion.
A similar method is described in U.S. Pat. No. 4,744,504 (Turner). Similar to the method described in the '660 patent described supra, in this patented method, the internal surface of the host is plated with a low melting point bonding metal alloy. A cylindrical cladding member is placed inside the plated cylindrical host, and then circumferentially welded on one end to the host. The other end is then welded to create a metallic gas reservoir. The annular space between the cladding member and the host is evacuated to remove water vapor and oxygen. The subassembly is heated to about 1650.degree. to about 2200.degree. F. to melt the bonding metal alloy. Finally, the subassembly is hot extruded to metallically bond the cladding member to the cylindrical host by means of liquid interface diffusion bonding. Unfortunately, the billet preparation for this process is fairly complicated due to the requirement for a very precise fit between the CRA and host components and the need to maintain a high level of vacuum between the layers which has to be maintained during heating up to extrusion temperature. The complexity of the method also makes it very expensive to implement. Variations and improvements on the patented Turner methods are further described in U.S. Pat. Nos. 5,000,368; 4,790,471; 4,765,529; 4,754,911; 4,881,679; and 4,869,422.
What is needed, then, is an inexpensive, commercially viable method of producing a high-integrity clad tubular product for a wide range of materials and sizes.